Modern communication appliances frequently contain a baseband unit and a radio-frequency unit, connected to it, in their transmitters. The baseband unit is used to preprocess the data to be transmitted. This includes, inter alia, modulation of the phase and amplitude of the data on a so-called baseband signal. The type of modulation used for modulation is in this case dependent on the chosen mobile communication standard. Examples of communication standards such as these are WCDMA/UMTS, GSM, EDGE, Bluetooth, 802.11 and HyperLan.
The baseband signal produced in this way in this case contains a real component I as well as a quadrature component Q, and is accordingly also referred to as an I/Q signal. The sum of the two components contains the modulated signal. In a final step, the digital baseband signal from the baseband unit is converted by means of a digital/analog converter to an analog baseband signal, and is emitted at the output of the baseband unit. In the following text, the expressions analog signal or analog component are intended to mean a signal or component which has a continuous value or is continuous over time. A digital signal or a digital component corresponds to a signal or component which has a discrete value or is present for a discrete time.
A radio-frequency unit which is connected to the output of the baseband unit adds the two components I and Q of the baseband signal and modulates them onto a carrier signal at the desired output frequency. The signal is suitably amplified in the radio-frequency unit, and is then emitted via an antenna.
In practice, the baseband unit and the radio-frequency unit are in the form of integrated circuits in two different semiconductor bodies, and are coupled to one another in a further step. This has the advantage that the baseband unit and radio-frequency unit can be developed separately from one another in order to take account of different conditions during manufacture, different requirements and different operating parameters. In addition, it is possible to use different manufacturing techniques and circuit techniques optimized to radio-frequency and baseband signal processing.
Analog circuits, in particular at the interfaces between the baseband unit and the radio-frequency unit, can lead to distortion and interference in the baseband signal that is produced. Interference and distortion such as this is referred to as an IQ signal error (“ISE” for “IQ signal error”). These have a direct influence on the output accuracy of the overall system. It is thus necessary to take account of any possible signal error even during the development of the circuits. This is done, for example, by derating individual components in order to compensate for the signal fluctuations caused by the interference. However, any enlargement increases the power consumption of a circuit, and the space that it occupies. If, furthermore, the baseband unit and the radio-frequency unit have different design requirements, the IQ signal inaccuracies caused in the interface could be sufficiently great that it is no longer possible to satisfy the desired performance parameters of the overall system.